Hemorrhoids are arteriovenous vascular pads lying circular above the dentate line and granting the "fine" continence of the rectum. Anatomically, the y are called Corpus cavernosum recti. The blood supply is taken from branches of the superior rectal artery. This explains the predilection sites for hemorrhoids on the rectal wall at 3, 7 and 11 o'clock in lithotomy position.
The blood flow is obstructed by the ension of the inner anal sphincter muscle - the hemorrhoids fill up like a sponge and tighten the rectum against fluids and gases.
This is called a hemorrhoidal disease - in common language unprecisely just called "hemorrhoids" - when the hemorrhoidal blood pad is enlarged and sympoms appear.